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Reprinted from the Distributor's Link Magazine, Winter 1997

About the Author/WAYNE WALLACE

Wayne Wallace is the president of Applied Bolting Technology Products. Thecompany provides bolting consulting services and manufactures direct tensionindicating washers from its base in Ludlow, Vermont. Wallace is a member of theResearch Council on Structural Connections, a director of the Bolting Technol-ogy Council, and author of numerous papers on the practical aspects ofqualityassurance in structural bolting.

Anti-Corrosion Coatings

For Bolts and DTIs

Bolts and DTls are used in bridges, refineries, roller

coasters, towers, and such structures; in the pol-

luted big city areas of the eastern US and in sea-

coast situations. In these locations good quality anti-

corrosion coatings are mandatory. Inorganic and or-

ganic zinc based paints with specific types of so-

phisticated topcoats are used to protect the steel

substrate, and you cant put plain old uncoated bare

steel bolts in the connections that hold these struc-

tures together.

Because were bolting specialists, even though notcorrosion experts, Applied Bolting Technology gets

questions about what coatings to use on bolts and

DTls. Heres how we answered some of them, with

advice from others. We though readers of Link might

appreciate a synopsis.

Mechanical zinc is superior to

hut dipped zinc at least when

topcoated, so it is not worthwhile

to hot dip them.

Subject - Expected Relative CorrosionResistance of Hot Dipped vs.Mechanically Galvanized Bolting Hardware

Question: You only offer mechanically galvanized

DTls, and we have installed your DTls on bolts

which have been mechanically galvanized (MG) to

ASTM B695 C1 50 on a water treatment plant. The

specifications, however, called for hot dipped galva-

nized (HDG) bolts and DTls to ASTM Al53 Class

C.

Will the corrosion life of the bolts be adversely af-

fected by the substitution of mechanically galvanized

bolts and DTls for hot dipped galvanized hardware?

Answer: No. When coated, the MG hardware might

even be superior. Given that the coating thicknessof the MG and HDG zinc substrate is equivalent in

these specifications, both systems should demon-

strate a similar corrosion resistance. The corrosion

tests comparing the two types of zinc coatings on

bolt show that, in a specific type of accelerated cor-

rosion test1, MG OUTPERFORMED HDG ZINC by a

wide margin to first red rust.

Background: Until very recently we have only

made mechanically galvanized DTls. Its been sometime since hot dipped zinc has been attempted on

DTls, and is currently not offered. Mechanical zinc

is superior to hot dipped zinc at least when topcoated,

so it is not worthwhile to hot dip them. If a customer

insisted that we supply hot dipped DTls, we would

oblige and try it, and of course only sell them if the

load/gap properties were still predictable after hotdipping. Mechanically galvanized DTls, however, will

function correctly with hot dipped bolts provided thecombination is proven on a Skidmore prior to instal-

lation. On the other hand, you must not mix HDG

nuts, for example, with MG bolts, because of the

possibility of encountering an uninstallable assem-

blyAccording to Thomas Cook, who is a chemist and

professor at Black Hills University, MG zinc tends to

be looser in nature than HDG zinc, and as a sub-

strate for paint this may be advantageous. He fur-

ther states that MG bolts tend to have more zinc on

their shanks than do HDG bolts (thats good). Then

he goes on to point out that HDG nuts are tapped

after galvanizing so they can be assembled, and their

uncoated ID can allow a corrosion cell to developjust ahead of the nut (thats bad). MG nuts are, on

the other hand, tapped oversized before coating, and

so will have some zinc on the ID.

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ANTI-CORROSION COATINGS,

According to Simon Boocock of the Steel Struc-

tures Painting Council, if the zinc thickness depos-

ited is similar, then there should be reasonable

agreement on corrosion life when topcoated.

Allan Stoneman of ILZRO (International lead-Zinc

Research Organization) cautions that the Rome Pa-

per published in 1988 compared HDG with equiva-lent thickness plated zinc (not MG zinc) and found

plated zinc definitely inferior when exposed to se-

vere industrial atmospheres.

Gordon Allison of McDermid, Inc., states categori-

cally that an MG substrate cannot be inferior to HDG,

and in most sulfur dioxide corrosion tests MG per-

forms much better than HDG.

Subject - The DTI Gap As ACorrosion Crevice

Question:This is 1996, and DTls have been usedin coastal environments and in corrosion-prone re-

fineries for more than 25 years. Is there any evidence

that leaving a DTI gap of, for example, .005 maxi-

mum, poses any corrosion threat to the bolt when

the structure has been painted?

Answer: No, there is no evidence of this at all. In

fact, there is considerable evidence to the contrary.

The residual DTI gap, which was in evidence imme-

diately after the bolt installation, is really not there

after a short while. It gets filled up, or bridged, by

the paint system, and facility owners report no dif-

ference in corrosion potential or maintenance require-ments between a bolted connection with and with-

out DTls.

Background: Intellectually, it is possible to con-

ceive of a bolt corrosion problem that stems from

the hypothetical situation where the DTI residual gap

allows the products of corrosion to somehow pass

through to the bolt shank. But think of all the struc-

tures constructed around tidal estuaries in England

starting in the 1960s. Think of the thousands of

bridges constructed in the coastal areas of the US

like Oregon, California, South Carolina, Alaska, andaround New York City (the George Washington

Bridge, Goethals Bridge, etc.). There has been no

documented occurrence of any special corrosion situ-

ation that has been linked to the presence of the

hundred million or so DTls installed in those struc-

tures. Even uncoated weathering Type 3 bolts that

are installed in combination with epoxy coated DTls

have not shown any signs of distress due to corro-

sion from the apparent residual DTI gap.

Parenthetically, this author has had personal ex-perience in the reconstruction of a 75-year-old, com-

pletely uncoated, riveted railroad bridge in suburban

Toronto. The areas around the rivets, the rivets them-

selves, and the plies within the clamp of the rivet, all

were seen to be sound, even though the steel angles

through which the rivets passed were, after 75 years,

substantially corroded away. By substantially cor-

roded away, I mean that they practically werent

there, except around the rivets. Clearly, the rivets

provided some low level of clamping force all those

years, and kept the products of corrosion from en-

tering between the plies, just like a correctly

tensioned bolt will do. And its DTls that best as-sure the correct clamping force which will keep the

connection plies together.

Subject - Better Corrosion Protection

Than ZincQuestion: If corrosion is a serious problem, what

coating(s) are commercially and economically avail-

able for bolts and DTls that provide superior perfor-

mance to that of zinc, and that will not complicate

the nut/bolt thread fit and the torque resistance of

the assembly during tightening? And, as a corollary,is there a coating that is suitable for A490 bolts and

DTls since zinc is not allowed?

Answer:There is at least one, but they all go un-

der the general name of bi-metallic coatings. There

are a number of them around, but the most visible is

Dacromet, which is a combination zinc and alumi-

num coating of thickness about a half mil or less.

Dacromet can be applied to both A325 and A490

bolts and DTls, will provide from two to five times

the corrosion resistance of MG, costs about the

same, and can be topcoated by any means or byDacromets special line of coatings for even better

corrosion resistance. In our opinion, if you can coat

your bolts, nuts, washers, and DTls with something

that gets about five times the corrosion resistance

of zinc, at the same costs, its a no-brainer. Go for it.

Coating A490 bolts with Dacromet is a real ad-

vantage to engineers faced, for example, with reno-

vating bridge connections for increased capacity,

such as to the new seismic codes. These engineers

want to use A490 bolts as a substitution for the same

diameter A325 bolts. Their alternative is to put larger

diameter A325 bolts into the connection, which ne-cessitates drilling out the holes, refitting new plates

if that results in bolt spacing problems, etc., all of

which is very costly. Galvanized A490 bolts and theirnuts are currently prohibited, so a Dacromet coated

A490 is a natural. 0

1 Pittsburgh Testing Laboratory, Tests for 3M Company,Salt Spray, 1978 (Holford)

Twin City Testing, Tests for 3M Company, Salt Spray, 1982(Holford)